This application claims the priority of Korean Patent Application No. 2003-20738, filed on Apr. 2, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The invention relates to an active matrix type flat panel display including a thin film transistor (TFT), and more particularly, to a flat panel display including a TFT having a polycrystalline silicon as an active layer, and different crystallization structures for the channel areas of the active layers of a switching TFT and a driving TFT.
2. Description of the Related Art
A thin film transistor (TFT) in a flat display device such as a liquid display device, an organic electroluminescence display device, or an inorganic electroluminescence display device is used as a switching device for controlling operations of pixels and as a driving device for driving the pixels.
The TFT includes a semiconductor active layer having a drain area and a source area which are doped with a high concentration of impurities and a channel area formed between the drain area and the source area, a gate insulating layer formed on the semiconductor active layer, and a gate electrode formed on the gate insulating layer which is located on an upper part of the channel area of the active layer. The semiconductor active layer can be classified as an amorphous silicon or a polycrystalline silicon according to the crystallized status of the silicon.
A TFT using amorphous silicon is advantageous in that a deposition can be performed at a low temperature, however, it is disadvantageous in that an electrical property and a reliability of the TFT are degraded. Also, it is difficult to make larger display devices. Thus, recently, polycrystalline silicon is being used. Polycrystalline silicon has a higher mobility of about tens to hundreds of cm2/V.s, and low high frequency operation property and leakage current value. Thus, polycrystalline silicon is suitable for use in large-sized flat panel displays of high resolution.
A TFT is used as the switching device or the driving device of the pixel in the flat panel display, as described above. An organic electroluminescence display device of an active matrix type with an active driving method includes at least two TFTs per sub-pixel.
The organic electroluminescence device has an emission layer made of an organic material between an anode electrode and a cathode electrode. In the organic electroluminescence device, when a positive voltage and a negative voltage are respectively applied to the electrodes, holes injected from the anode electrode are moved to the emission layer through a hole transport layer, and electrons are injected into the emission layer through an electron transport layer from the cathode electrode. The holes and electrons are recombined on the emission layer to produce exitons. The exitons are changed from an excited status to a ground status, and accordingly, phosphor molecules in the emission layer are radiated to form an image. In case of a full-color electroluminescence display, pixels radiating red (R), green (G), and blue (B) colors are disposed as electroluminescence devices to realize the full colors.
In the active matrix type organic electroluminescence display device, a panel with high resolution is required, however, the above described TFT formed using the polycrystalline silicon of high function causes some problems in this case.
That is, in the active matrix type flat panel display device such as the active matrix type organic electroluminescence display device, the switching TFT and the driving TFT are made of the polycrystalline silicon. Thus, the switching TFT and the driving TFT have the same current mobility. Therefore, switching properties of the switching TFT and low current driving properties of the driving TFT cannot be satisfied simultaneously. That is, when the driving TFT and the switching TFT of a high resolution display device are fabricated using the polycrystalline silicon, which has a having larger current mobility, the high switching property of the switching TFT can be obtained, however, the brightness becomes too bright because an amount of current flowing toward an electroluminescence (EL) device through the driving TFT increases. Thus a current density per unit area of the device is increased while a life time of the EL device is decreased.
On the other hand, when the switching TFT and the driving TFT of the display device are fabricated using the amorphous silicon, which has a low current mobility, the TFTs should be fabricated in such way that the driving TFT uses a small current and the switching TFT uses a large current.
To solve the above problems, methods for restricting current flowing through the driving TFT are provided, such as, a method for increasing resistance of a channel area by reducing a ratio of a length to a width of the driving TFT (W/L) and a method for increasing resistance by forming a low doped area on the source/drain areas of the driving TFT.
However, in the method decreasing the W/L by increasing the length, a length of the channel area increases, thus forming stripes on the channel area and reducing an aperture area in a crystallization process in an excimer laser annealing (ELA) method. The method decreasing W/L by reducing the width is limited by a design rule of a photolithography process, and it is difficult to ensure a reliability of the TFT.
Also, the method for increasing the resistance by forming the low doped area requires an additional doping process.
A method for increasing TFT properties by reducing a thickness of the channel area is disclosed in U.S. Pat. No. 6,337,232.
The method for reducing a ratio of a length for a width of the driving TFT is disclosed in Japanese Patent Publication No. 2001-109399.